1. Field of the Invention
The present invention relates to a method for measuring a variable bandwidth wireless channel, and a transmitter and a receiver therefor; and, more particularly, to a method for measuring variable bandwidth wireless channel by creating a variable bandwidth signal by fixing a digital-to-analog (D/A) operation speed and varying over-sampling according to a measurement bandwidth in a transmitting part, and acquiring a variable bandwidth signal by varying a receiving intermediate frequency and an analog-to-digital (A/D) operation speed according to a measurement bandwidth in a receiving part, and a transmitter and a receiver therefor.
This work was supported by the IT R&D program for MIC/IITA [2005-S-001-02, “Development of Wireless Spatial Channel Model for next generation mobile communication”].
2. Description of Related Art
Information of a wireless channel characteristic is required in developing a next generation mobile communication system. A wireless channel should be measured first in order to grasp a characteristic of a wireless channel.
Since a wireless channel of only a single bandwidth is measured in a conventional wireless channel measurement method, a plurality of wireless channel sounders are required to measure a wireless channel of a plurality of bandwidths. To be specific, the conventional wireless channel measurement method will be described as follows.
FIG. 1 is a block diagram showing a transmitter and a receiver of a conventional channel sounder measuring a single bandwidth wireless channel.
The transmitting part includes pre-processings 111 to 115 and realtime-processings 116 to 117. The transmitting part transmits a probing signal to a wireless channel. In the pre-processing of the transmitting part, a probing sequence is created and stored in a memory 115 inside transmitting part. Herein, the probing sequence means a sequence where fixed over-sampling, band-limitation, and up-conversion into a transmitting intermediate frequency based on a digital intermediate frequency method are performed on an original sequence for measurement.
Subsequently, when the wireless channel is measured, the transmitting part performs conversion and filtering on the probing sequence stored in the memory 115 into an analog signal according to a fixed D/A operation speed through a real-time process, and finally transmits the probing signal, on which the frequency up-conversion and filtering are performed, as a carrier frequency.
The receiving part includes real-time processings 121 to 123 and post-processings 124 to 127. The receiving part acquires and analyzes a probing signal passing the wireless channel.
When the wireless channel is measured, the receiving part performs frequency down-conversion on the signal received in real-time processing into a fixed receiving intermediate frequency, converts the signal into digital data according to a fixed A/D operation speed, and stores the digital data in a memory of an A/D converter in real-time. When a predetermined quantity of data is acquired, the receiving part stores the digital data stored in the memory of the A/D converter 122 in own storage 123. The A/D converter 122 stops an operation while the digital data are stored in the storage and restarts the operation on a probing signal received after the digital data are stored in the storage. After the wireless channel is measured, the receiving part analyzes the data stored in own storage 123 by acquiring signals a plurality of times in wireless channel measurement through post-processing, and checks a characteristic of a wireless channel.
In the conventional wireless channel measurement method, it is assumed that the digital intermediate frequency method of Digital IF, 114 and 124 is applied to the transmitting part and the receiving part and a 4 times over-sampling 112 is applied. The digital intermediate frequency method is assumed because a receiving signal may be distorted when an analog intermediate frequency (analog IF) method is used in the receiving part targeting exactly acquiring the probing signal passing the wireless channel. That is, when the analog intermediate frequency method is adopted in the receiving part, the receiving signal is converted into an In-phase signal (I signal) and a quadrature-phase signal (Q signal) of a baseband from an intermediate frequency band in a form of an analog signal, distortion may occur due to I/Q mismatch in the above procedure.
A reason that the 4 times over-sampling is assumed in the transmitting part is as follows. The 4 times over-sampling 112 and a band-limited filter 113 are required to adopt the digital intermediate frequency method in the transmitting part. The over-sampling should be performed more than 4 times in order to prevent aliasing in a spectrum with no regard to a characteristic of the band-limited filter 113 in the digital intermediate frequency method. That is, when the over-sampling is performed less than 4 times, aliasing may occur with no regard to a kind of transmitting intermediate frequency according to the band-limited filter characteristic. When the over-sampling is performed more than 4 times, the transmitting intermediate frequency which does not generate aliasing with no regard to the band-limited filter characteristic is widely selected. However, since over-sampling increases, a D/A operation speed needs to be fast.
Therefore, 4 times over-sampling is selected since unnecessary over-sampling is not required in the transmitting part. And, when over-sampling is 4 time, the transmitting intermediate frequency should be ¼ of the D/A operation speed (f_D/A) in order to prevent aliasing in a spectrum with no regard to the band-limited filter characteristic. In this case, there is a benefit that the operation quantity decreases.
                                          (                          Digital              ⁢                                                          ⁢              IF              ⁢                                                          ⁢              process                        )                    =                      Object            ⁢                                                  ⁢                          sequence              ⁡                              (                n                )                                      *                          cos              ⁡                              (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    IF                                    ⁢                                      n                                          f                                              D                        /                        A                                                                                            )                                                    ⁢                                  ⁢                  where          ⁢                                                            ⁢                                                          ⁢          when                ⁢                                  ⁢                                            f              IF                        =                                          f                                  D                  /                  A                                            4                                ,                                    cos              ⁡                              (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    IF                                    ⁢                                      n                                          f                                              D                        /                        A                                                                                            )                                      =                          cos              ⁡                              (                                                      n                    ⁢                                                                                  ⁢                    π                                    2                                )                                                                        Equation        ⁢                                  ⁢        1            
In Equation 1, when the transmitting intermediate frequency is ¼ of the D/A operation speed,
  cos  ⁡      (                  n        ⁢                                  ⁢        π            2        )  is a repeat of [1,0,−1,0]. Accordingly, the operation quantity is small and the digital intermediate frequency method can be easily applied. In addition, the transmitting part may adopt the analog intermediate frequency method. However, in this case, the transmitting part also requires the over-sampling and the band-limited filter in order to decrease signal distortion due to the analog filter after D/A converter and the Sample And Hold (S/H) phenomenon of the D/A converter. Therefore, if the transmitting part requires the over-sampling and the band-limited filter all the time, it is good selection to adopt the digital intermediate frequency method as the receiving part does.
As described above, if 4 times over-sampling about the measurement bandwidth is adopted and the transmitting intermediate frequency is set at ¼ of the D/A operation speed, the transmitting intermediate frequency is set equally to the measurement bandwidth. A case that the measurement bandwidth is 100 MHz in the conventional single bandwidth wireless channel measurement transmitter and receiver is shown as follows.
TABLE 1Transmitting partReceiving partD/AA/DIntermediateoperationIntermediateoperationMeasurementfrequencyspeedOver-frequencyspeedbandwidth(f _TxIF)(f_ D/A)sampling(f _RxIF)(f_A/D)Decimation100 MHz100 MHz400 MHz4100 MHz400 MHz4
As described above, since the case that the measurement bandwidth is fixed is considered in the conventional wireless channel measurement method, there is a problem that a plurality of wireless channel measurement transmitters and receivers are required in order to measure the wireless channel of a plurality of bandwidths.